Atmospheric Chemistry Of Biogenic Hydrocarbons

A great variety of organic compounds are emitted by vegetation. These biogenic compounds are highly reactive in the atmosphere. They are basically alkenes or cycloalkenes, and their atmospheric chemistry is generally analogous to that of alkenes. Because of the presence of C=C double bonds these molecules are susceptible to attack by 03 and N03, in addition to the customary reaction with OH radicals.

TABLE 6.5 Estimated Tropospheric Lifetimes of Organic Compounds

Lifetime Against Reaction with

Lifetime Against Reaction with

TABLE 6.5 Estimated Tropospheric Lifetimes of Organic Compounds

OH"

NO3

hv

«-Butane

5.7 days

_

1.7 months

Propene

6.6h

1.6 days

5.9 h

Benzene

12 days

Toluene

2.4 days

1.1 month

m-Xylcne

7.4 h

10 days

Formaldehyde

1.5 days

4 days

4h

Acetaldeyde

11 h

20 h

5 days

Acetone

66 days

38 days

Isoprene

1.7 h

1.3 days

0.8 h

a-Pinene

3.4 h

4.6 h

6min

P-Pinene

2.3 h

1.1 days

15 min

Camphene

3.5 h

18 days

1.8 h

2-Carene

2.3 h

1.7 h

1.8 min

3-Carene

2.1 h

10h

3.3 min

d-Limonene

1.1 h

1.9 h

2.7 min

Terpinolene

49 min

17 min

0.4 min

"12-h daytime OH concentration of 1.5 x 106 molecules cm-3 (0.06 ppt). b24-h average 03 concentration of 7 x 10" molecules cm-3 (30ppb). c12-h average NO3 concentration of 4.8 x 108 molecules cm-3 (20ppt).

"12-h daytime OH concentration of 1.5 x 106 molecules cm-3 (0.06 ppt). b24-h average 03 concentration of 7 x 10" molecules cm-3 (30ppb). c12-h average NO3 concentration of 4.8 x 108 molecules cm-3 (20ppt).

Tropospheric lifetimes of organic species due to reaction with OH, N03, and 03 can be estimated by combining the rate constant data (Appendix B) with estimated ambient tropospheric concentrations of OH, N03, and 03. Resulting tropospheric lifetimes of a number of organic species, including several biogenic hydrocarbons, with respect to these gas-phase reactions are given in Table 6.5. An important point to note is that the atmospheric lifetimes of the biogenic hydrocarbons are relatively short compared to those of other organic species. The OH radical and ozone reactions are estimated to be of generally comparable importance during the daytime, and the N03 radical reaction is important at night.

If one had to single out the most important biogenic hydrocarbon in atmospheric chemistry, it would be isoprene. Isoprene has the chemical formula

CH3 1

CH2=C-CH=CH2

Another name for isoprene is 2-methy 1-1,3-butadiene. When illustrating the reactions of isoprene it is convenient to use the shorthand chemical structure (see Table 2.12), where the double bonds are indicated by double lines, and each vertex and the end of each line indicates a carbon atom with the requisite number of hydrogen atoms. Isoprene reacts with OH radicals, N03 radicals, and 03 (Paulson et al. 1992a,b; Paulson and Seinfeld atmospheric chemistry of biogenic hydrocarbons

1992; Atkinson and Arey 2003) and rate constants for these reactions are known (see Tables B.9 and B.10).

The OH-isoprene reaction proceeds almost entirely by addition of the OH radical to the C=C double bonds. Formaldehyde, methacrolein (CH2=C(CH3)CHO), and methyl vinyl ketone (CH2=CHC(0)CH3) have been identified in the laboratory as major products of the OH-isoprene reaction. Figure 6.18 shows the initial steps of OH attack on h,c h2c ©

no n02 no n02 no no2

no no2

(1-2 addition)

(1-2 addition)

(2-1 addition)

(2-1 addition)

(3-4 addition)

(4-3 addition)

(4-1 addition)

FIGURE 6.18 Isoprene-OH reaction mechanism. OH can add to four different positions in the isoprene molecule, denoted 1-4. We show the pathways from OH addition, followed by 02 and NO reaction, leading to six alkoxy radicals (I-VI). Nitrate formation in the NO reaction step is not shown.

CH,OH

u.d.

H,C 1

Methyl vinyl ketone

Methyl vinyl ketone

Formaldehyde

OH H

Methacrolein

Carbonyl products

H3C 1

CH II

Methacrolein

Carbonyl products figure 6.18 (Continued)

isoprene. The result, after 02 addition and reaction with NO, are the six alkoxy radicals denoted I-VI. Addition of OH to the terminal carbon atoms (1 or 4) produces allylic radicals, to which subsequent addition of 02 may occur at carbon atoms either [3- or 8- to the OH group. (3-Addition leads to radicals I-IV; §- addition leads to radicals V and VI. Figure 6.18 shows the atmospheric fates of the six alkoxy radicals. Radicals I-IV undergo unimolecular decomposition (UD); V and VI isomerize or react with 02. The major products, methacrolein or or ch2=c-ch

CH3 O

i m and methyl vinyl ketone

or ch3-c-ch=ch2

m are indicated.

The 03-isoprene reaction proceeds by initial addition of 03 to the C=C double bonds to form two primary ozonides, each of which decomposes to two sets of carbonyl plus biradical products (a minor channel is apparently formation of a 1,2-epoxymethyl butene). This mechanism is consistent with the formation of formaldehyde, methacrolein, and methyl vinyl ketone. As in other 03-alkene reactions, OH radicals are observed in significant yield, about 0.27 molecule of OH per 03-isoprene reaction.

The N03-isoprene reaction proceeds by N03 addition to the C=C double bonds, with addition at position 1 dominating over that at position 4. Consistent with laboratory product data, N03 + isoprene involves the formation of OOCH2CH=C(CH3) CH20N02, which, in the presence of NO, forms the corresponding alkoxy radical. This alkoxy radical can react with 02 forming H02 and CH3C(CH20N02)=CHCH0, isomerize by H-atom abstraction from the —CH20N02 group, or isomerize by H-atom abstraction from the —CH3 group, giving rise to C5-hydroxynitrato carbonyls or formaldehyde plus a C4-nitrato carbonyl.

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